Human cytomegalovirus (hCMV) is a herpesvirus that persists indefinitely by way of a latent infection. Up to 99% of the world's population is latently infected with the virus. Reactivation from latency causes significant morbidity and mortality in immune compromised individuals. Coexistence of latent hCMV in the host poses increased risk of age-related pathologies, including heart disease. There is no vaccine for hCMV and existing antivirals do not clear the virus because they fail to target the latent virus. The mechanisms underlying hCMV latency are poorly understood. Defining these mechanisms is critical to ultimately controlling or eradicating hCMV. During the previous funding period, the UL138 protein, pUL138, was define as the first virus-coded determinant of hCMV latency. pUL138 is encoded by a polycistronic locus, termed the UL133/8 locus, which coordinates the expression of three additional, previously unstudied proteins. Surprisingly, one of these proteins, pUL135, antagonizes the action of pUL138. Thus, while pUL138 suppresses virus replication for latency, pUL135 overcomes pUL138-enforced latency to promote reactivation. Based on our discoveries, we propose a new model whereby pUL135 and pUL138 form a switch that actively drives infection towards latency or reactivation. The goal for this renewal application is to defin the mechanisms by which the UL135/UL138 switch functions. To this end, we have identified cellular interacting partners for pUL138 and pUL135. Importantly, pUL135 and pUL138 each interact with cellular regulators of ubiquitination and protein trafficking that result in opposing effects on receptor tyrosine kinase (RTK) expression at the cell surface. Our findings allow us to test the hypothesis that pUL135 and pUL138 antagonize one another to balance latency and reactivation by exerting opposite effects on cellular ubiquitination and protein trafficking pathways. In the next funding period, we will define the molecular interplay between pUL135 and pUL138 that tips the balance between replicative or latent states of infection. Further, we will determine the mechanisms by which pUL135 and pUL138 regulate protein trafficking during infection and how this virus modulation of cellular trafficking pathways impacts RTK function and viral latency and reactivation. This work will establish a framework within which to identify novel targets for antivirals targeting latent reservoirs-the key to controlling or eradicating hCMV.